It is known that most fish kept in aquaria are of tropical origin. The living conditions for such animal species are marked by temperatures which on average lie between 22 and 26° C.
Temperatures above or below the abovementioned range may be fatal to fish living in aquaria.
Electric heaters or temperature regulators are generally used to maintain this temperature range, and these essentially comprise a glass bulb in which an electrical resistance and a thermostat, that is a component which detects the temperature of the water and compares it with the reference value to open and close the electricity supply circuit to the resistance, are hermetically housed.
Thermostats of the electronic type which have a good quality sensor but an excessive cost, which greatly limits their application, are known.
Another known type of thermostat is that of the bi-metal strip in which the switch which brings about opening and closing of the supply circuit comprises a pair of metal strips having different thermal expansion coefficients adjacent and joined together in such a way as to form a single unit.
This type of switch has one end attached to one of the terminals of the electricity supply line, and the other end is provided with a moving contact which selectively joins together with a fixed contact attached to the other terminal of the same line.
As the temperature changes the differential expansion of the two strips causes a change in the curvature of the strip along its length and therefore a different relative position between the moving contact and the fixed contact so as to close or open the supply circuit to the resistance.
In turn, bi-metal thermostats may be subdivided into thermostats having a fixed setting, in which setting is generally performed in the factory to a fixed value, for example 24° C., which cannot be changed by the user in any way. The electrical supply circuit to the resistance is opened and closed at this value and therefore this type of bi-metal thermostat is unable to optimise adjustment within the desired temperature range.
Another type of bi-metal thermostat is that having simple control, without a graduated scale, similar to the above but in which action has to be taken on the thermostat using an external thermometer in order to regulate its operating temperature, making the regulating operation extremely complex.
Bi-metal thermostats with a graduated scale make it possible to set the temperature by positioning an indicator on a graduated thermometric scale, achieving the temperature desired. In these known thermostats the user can move an indicator along the thermometric scale until it reaches a position corresponding to the desired temperature. At that position the electricity supply circuit to the resistance will be open or closed so as to bring the temperature of the water close to that set on the indicator, within a particular range of oscillation.
Some examples of bi-metal thermostat heaters are described and illustrated in U.S. Pat. No. 4,315,143, U.S. Pat. No. 4,327,281, U.S. Pat. No. 4,975,562 and EP-A-757,507.
A particular disadvantage in known bi-metal regulators lies in the fact that when the circuit is closed the current passes through the strips themselves and as a result of the Joule effect brings about a change in sensitivity depending upon the power of the heating resistance. This gives rise to some slowness in reaching temperature equilibrium conditions and therefore less accuracy in the regulator.
However, users normally need to bring an aquarium rapidly to the desired temperature and then hold it constant without peaks or drift, which are lethal for the survival conditions of the species living in the aquarium.